The U.S. Department of Justice is preparing to sue Citigroup Inc on charges that the bank defrauded investors on billions of dollars worth of mortgage securities in the run-up to the financial crisis. A lawsuit could be filed in U.S. District Court in Brooklyn as early as next week, involving penalty numbers of $10 billion or more. Bloomberg News reported that the Justice Department asked the bank to pay more than $10 billion, and that the bank had offered less than $4 billion.